Its a captive audience.
8 million baptised dubs get a copy each, plus another 6 or so on average for placement. Theres your 40- 50 million without even trying.
Now, how many of these are actually read cover to cover - even by JW's?
Of the non- JW's who receive a free copy, how many of them would leave their house and go buy a copy if that was the only way to get it? Probably zero.